A non-sense mutation is a mutation that results in a pre-mature stop codon. The premature stop codon results in a tuncated version of a protein, rendering it inefficient or useless. There are several diseases associated with non-sense mutations, including; cystic fibrosis and beta-thalassaemia.
Example of a Nonsense mutation. Complements of the NIH genetic home reference.
References:
NIH Genetic Home Reference: Non-Sense Mutation